Kent Sorenson, an Iowa state senator who had served as Bachmann's state campaign chairman for nearly a year, said he had decided to switch his support to Paul because the campaign had reached "a turning point."

"When the Republican establishment is going to be coming after Ron Paul, I thought it is my duty to come to his aid," Sorenson said, announcing his endorsement for the Texas congressman during a rally at the Iowa State Fairgrounds.

Sorenson said in a statement that Paul was "easily the most conservative" member of the top tier in the race for the Republican nomination to challenge Democratic President Barack Obama in elections in November next year.

"The fact that he doesn't take this decision lightly tells a great deal about the senator and Ron Paul," said Jesse Benton, Paul's national campaign chair.

Paul has a strong organization in the early voting state and is one of the favorites to win the Iowa caucuses vote on January 3.

Sorenson's defection gave the Paul campaign some respite from questions about his links to newsletters two decades ago that carried his name and contained racist, anti-homosexual and anti-Israel rants.

Soon after Paul took the stage at the rally late Wednesday, he was interrupted by a few protesters from the "Occupy Des Moines" movement.

"Freedom of speech. Ain't it wonderful?," Paul said, "We're all upset and we want a change in Washington. As a matter of fact, that's what our purpose is."